In grinding machines, care must be taken that the maximum permissible speed of the grinding wheel is not exceeded. In a conventional grinding machine of the above-described type, the manner of preventing impermissibly high spindle speeds includes the regulating member of the speed regulator being returned automatically after each stopping of the spindle to a minimum speed determined by the largest grinding wheel. After a grinding wheel exchange, the operator must, prior to increasing motor speed, inform himself through a table which contains the permissible speed ranges for various grinding wheel diameters of the maximum permissible speed for the grinding wheel just mounted. Only then is he permitted to increase the motor speed by means of a regulating potentiometer, but not beyond the maximum permissible speed. Simultaneous with the grinding wheel exchange, respective protective hoods corresponding to the shape and diameter of the respective grinding wheels and mountable on the spindle housing are exchanged. These safety precautions, however, depend entirely on the conscientiousness of the operator and are no longer considered sufficient today.
Therefore, a basic purpose of the present invention is to provide a device for automatically limiting the steplessly adjustable speed of a drive motor for the spindle of a grinding machine to a speed which is the maximum permissible speed for the size of the grinding wheel currently being used, which grinding machine has a simple and reliable structure and avoids exceeding a maximum permissible speed in a manner independent of the conscientiousness of the operator.